EU has released a paper today
https://ec.europa.eu/info/sites/info/files/communication-preparing-withdrawal-brexit-preparedness.pdf
Main consequences of UK leaving the EU without a Withdrawal Agreement include (see page numbered 5):
[TABLE=width: 556]
[TR]
[TD]Border issues: The European Union must apply its regulation and tariffs at borders with the United Kingdom as a third country, including checks and controls for customs, sanitary and phytosanitary standards and verification of compliance with EU norms. Transport between the United Kingdom and the European Union would be severely impacted. Customs, sanitary and phytosanitary controls at borders could cause significant delays, e.g. in road transport, and difficulties for ports.
[/TD]
[/TR]
[/TABLE]